Note: Descriptions are shown in the official language in which they were submitted.
CA 02630986 2014-06-05
TWO VESSEL REACTOR SYSTEM AND METHOD FOR HYDROLYSIS AND
DIGESTION OF WOOD CHIPS WITH CHEMICAL ENHANCED WASH
METHOD
RELATED APPLICATION
[0001]
BACKGROUND OF THE INVENTION
[0002] This
invention relates to a method and an
apparatus for hydrolysis treatment of cellulosic fiber
material.
[0003] In
conventional systems, wood chips (or other
cellulosic or fiber material) undergo hydrolysis in a
first reactor vessel prior to introduction to a second
vessel, e.g., a digester. One such conventional system is
described in US Patent 4,174,997 ('997 Patent). In the
first reactor vessel, hydrolysis of the slurry of wood
chips passing through that vessel occurs under acidic
conditions. In the
first reactor vessel, hydrolysate,
e.g., sugars such pentose and hexose, is extracted from
wood chips and the hydrolysate is recovered. Fiber
material is discharged from the bottom of the first
reactor vessel and transferred via the transfer line to
the top of the second reactor vessel, e.g., digester, for
cooking treatment of the cellulosic material.
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[0004] In conventional systems, such as described in
the '997 patent, hydrolysis occurs throughout the first
reactor vessel. A chip slurry is introduced into the top
of the first reactor vessel and is discharged from the
bottom of the vessel. Heat is added to the vessel by
introducing hot water, e.g., 150 C degrees Celsius ( C),
to the bottom of the vessel and steam at the top of the
vessel. In addition, acidic solutions were added to
promote hydrolysis, especially where the material was at
temperatures below 150 C. The hot water flows upward in
the vessel, which is counter to the downward flow of
fiber material. The hot water and steam provide
sufficient heat to the material to maintain hydrolysis
through the vessel.
[0005] In some conventional systems, cooking chemical
such as white liquor, is introduced to the bottom of the
first reactor vessel and into a transfer pipe for
transporting the chip slurry from the first reactor
vessel to the second reactor vessel. The injection of
cooking chemicals to the bottom of the first reactor
vessel starts the impregnation of the fibers of the
cellulosic material in the bottom of the first reactor
vessel while the hydrolysis reaction is still underway.
It is undesirable to introduce cooking chemicals to the
cellulosic material while hydrolysis is ongoing.
BRIEF DESCRIPTION OF THE INVENTION
[0006] A novel hydrolysis system has been developed
for a pulping system. Cellulosic material, e.g., wood
chips, undergo hydrolysis in an upper region of a first
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vessel (hydrolysis reactor). Hydrolysis is preferably
conducted where the material in the vessel is at a
temperature of between 150 c and 175 C, more between 160 C
to 170 C.
Hydrolysis is preferably conducted where the
material in the vessel is preferably at a pH of 1 to 6,
and more preferably at a pH 3 to 4.
Hydrolysate and
liquids are removed from the hydrolysis reactor through
an extraction screen.
[0007] Below
the extraction screen, cool wash liquid
flows upward through a wash zone in the hydrolysis
reactor and to the extraction screen. The cool
wash
liquid suppresses hydrolysis reactions in the cellulosic
material below the extraction screen. Substantially all
of the hydrolysis is preferably performed above the
extraction screen in the hydrolysis reactor. The cool
wash liquid preferably has a temperature of 10 C to 70 C
cooler than the hydrolysis temperature, more preferably
20 C to 50 C cooler, and most preferably 25 C to 35 C
cooler than the hydrolysis temperature. The cool
wash
liquid preferably has a pH of 3 to 7,
and most
preferably a pH of 4 to 5. Further the cool wash liquid
preferably includes mostly water and may include an added
chemical in an amount of 0.01 percent (%) to 5 percent of
the amount of cellulosic material, e.g. wood, in the
slurry flowing through the vessel. The
amount of added
chemical is most preferably 0.1 percent to 1 percent of
the amount of cellulosic material in the slurry. The
chemical added to the cool wash water may be either or
both sodium hydroxide (NaOH) or essentially sulfur free
white liquor to produce a cool wash liquid.
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[0008] A
reactor vessel system has been developed
comprising: a first reactor vessel having a material
input receiving cellulosic material and a material
discharge for the cellulosic material, wherein the
cellulosic material flows through the first reactor
vessel from the material input to the material discharge;
a hydrolysate and liquid extraction screen in the first
reactor vessel; a first region of the first reactor
vessel between the material input and the liquid
extraction screen, wherein the first region is maintained
at conditions promoting a hydrolysis reaction in the
cellulosic material; a heat energy inlet port for
introducing a heated fluid added to the cellulosic
material in or above the first region; a second region of
the first reactor vessel between the liquid extraction
screen and the material discharge in which the hydrolysis
is substantially suppressed; a wash liquid inlet port for
introducing a wash liquid below the extraction screen and
flowing through the second region to the extraction
screen, wherein the wash liquid is introduced at a
temperature below a hydrolysis temperature and the wash
liquid suppresses the hydrolysis second region; a
transport pipe having an inlet coupled to the material
discharge of the first reactor vessel and an outlet
coupled to a second reactor vessel, wherein the
cellulosic material flows from the material discharge,
through the transport pipe to the second reactor vessel,
and the second reactor vessel applies a cooking liquor to
the cellulosic material in the second reactor vessel, and
the second reactor vessel includes a liquid discharge
that extracts a portion of liquid from the second reactor
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vessel and directs the portion of liquid to at least one
of a lower inlet of the first reactor vessel or to the
transport pipe.
[0009] A flash tank may receive liquid extracted from
the extraction screen(s) of the first reactor vessel and
provide steam to the vessel at or above the first vessel
=
region. The flash tank may also discharge hydrolysate to
a hydrolysate recovery system.
[0010] A reactor vessel system has been developed
comprising: first reactor vessel having an upper material
input receiving cellulosic material and a bottom material
discharge for the cellulosic material, wherein the
cellulosic material flows through the first reactor
vessel from the material input to the material discharge;
a hydrolysate and liquid extraction screen in the first
reactor vessel; an upper region of the first reactor
vessel between the material input and the liquid
extraction screen, wherein the upper region is
maintained at or above a hydrolysis temperature at which
a hydrolysis reaction occurs in the cellulosic material;
a heat energy inlet port for introducing a heated fluid
to the cellulosic material in the upper region of the
first reactor vessel; a lower region of the first reactor
vessel between the liquid extraction screen and the
bottom material discharge in which the hydrolysis is
substantially suppressed; a wash liquid inlet port at a
lower region of the first reactor vessel for introducing
sufficient wash liquid to the vessel such that the wash
liquid flows up through the lower region to the
extraction screen, wherein the wash liquid is introduced
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at a temperature below the hydrolysis temperature and the
wash liquid cools and suppresses the hydrolysis reactions
in the second region of the reactor vessel; a transport
pipe having an inlet coupled to the material discharge of
the first reactor vessel and an outlet coupled to a
second reactor vessel, wherein the cellulosic material
flows from the bottom material discharge, through the
transport pipe to an upper inlet of the second reactor
vessel, and the second reactor vessel applies a cooking
liquor to the cellulosic material in the second reactor
vessel, and the second reactor vessel includes a liquid
discharge that extracts a portion of liquid from the
second reactor vessel and directs the portion of liquid
to at least one of a lower inlet of the first reactor
vessel or to the transport pipe.
[0011] A
processing system has been developed for
converting cellulosic material to pulp, the system
comprising: a first pressurized reactor vessel operating
at a pressure above atmospheric pressure, the first
reactor vessel including a material input receiving
cellulosic material and a material discharge for the
material, wherein the cellulosic material flows from the
material input to the material discharge, a heat energy
input port in an upper portion of the first reactor
vessel, a first region of the first reactor vessel
between the material input and a liquid extraction
screen, wherein the first region is maintained at a
hydrolysis temperature of at least 170 degrees Celsius in
the cellulosic material, the extraction screen having an
outlet for extracting hydrolysate and liquid from the
first vessel, and a second region of the first reactor
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between the liquid extraction screen and the discharge in
which a temperature is below the hydrolysis temperature
and the hydrolysis reactor is substantially suppressed
and a discharge of the first vessel below the second
region; the processing system further comprises a
transport pipe providing a flow conduit from the
discharge to a continuous digesting vessel, and the
continuous digesting vessel receives the cellulosic
material discharged from the first reactor vessel.
[0012] A method has been developed to produce pulp
from cellulosic material comprising: introducing
cellulosic material to an upper inlet in a first reactor
vessel; hydrolyzing the cellulosic material in upper
region of the an upper region of the first reactor vessel
by adding pressure and heat energy to the vessel;
extracting hydrolysate from the cellulosic material
through an extraction screen below the upper region and
in the first reactor vessel; introducing a wash liquid to
a lower region of the first reactor vessel where the wash
liquid suppresses hydrolysis of the cellulosic material
in the lower region and said wash liquid flows upward
through the cellulosic material to the extraction screen;
discharging the cellulosic material from a lower outlet
of the first reactor vessel; introducing the discharged
cellulosic material to a second reactor vessel, and
introducing cooking liquor into the top of the second
reactor vessel to digest the cellulosic material to
produce pulp.
[0013] A method has been developed to suppress
hydrolysis of cellulosic material comprising: introducing
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cellulosic material in an upper inlet of a first reactor
vessel, wherein the material moves downwardly through the
vessel; adding steam at above atmospheric pressure to the
first reactor vessel; maintaining at above a hydrolysis
temperature the cellulosic material in an upper region of
the first reactor vessel; extracting hydrolysate from the
cellulosic material through an extraction screen below
the upper region in the first reactor vessel; cooling the
cellulosic material below the extraction screen to a
temperature below the hydrolysis temperature, and
discharging the cellulosic material from a bottom outlet
of the first reactor vessel.
BRIEF DESCRIPTION OF THE DRAWING
[0014] FIGURE 1
is a schematic diagram of a continuous
pulping system having a chip feed, hydrolysis reactor and
a continuous digester reactor.
DETAILED DESCRIPTION OF THE INVENTION
[0015] In a two
reactor vessel system, steam is
introduced to the top of both vessels for heating and
pressurizing purposes. Hydrolysis occurs above extraction
screens in the top of the first reactor vessel. The
extraction screens in the first reactor vessel remove
hydrolysate as the wood chips or other cellulosic or
fiber material (collectively referred to cellulosic
material) introduced at the top of the first vessel
progress through the vessel and to a lower extraction
port of that vessel.
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[0016] The
cellulosic material is washed in the first
reactor vessel below the extraction screens. Wash liquid
is introduced at the bottom of the first reactor vessel
and flows upwards to the extraction screens. The wash
liquid may be water only or water mixed with one or more
chemicals, such as sodium hydroxide (NaOH) and
essentially sulfur free white liquor. The
diameter of
the first vessel may be uniform above and below the
extraction screen. The cellulosic material discharged
from the extraction port of the first reactor vessel is
introduced to the top of the second reactor vessel, which
may be a digester vessel. The
cellulosic material is
cooked in the second reactor vessel to generate pulp that
is discharged from a lower extraction port of the second
reactor vessel.
[0017] In the
first reactor vessel, the cellulosic
material is washed in a lower section of the vessel to
remove hydrolysate from the material. The washing in the
lower portion of the first vessel is performed with wash
liquid at a temperature below the hydrolysis temperature.
The wash liquid temperature is preferably 10 C to 70 C
cooler than the hydrolysis temperature, more preferably
20 C to 50 C cooler, and most preferably 25 C to 35 C
cooler than the hydrolysis temperature. The wash liquid
cools the cellulosic material to a temperature normal
hydrolysis temperatures. The cool
wash liquid flushes
out remaining hydrolysate from the cellulosic material,
lowers the temperature of the cellulosic material to
below the hydrolysis temperature, and adjusts the pH of
the cellulosic material to near or slightly above neutral
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(7 pH) in the first reactor vessel and prior to cooking
of the material in the second reactor vessel.
[0018] The cool wash liquid preferably has a pH of 3
to 7, and more preferably a pH of 4 to 5. Keeping the pH
of the cool wash liquid in these ranges prevents or
minimizes the precipitation of dissolved lignin in the
cooking chemicals of the second reactor vessel. The wash
liquid may include added chemicals, e.g., NaOH and
essentially sulfur free white liquor, to increase the
amount of hydrolysate extracted from the cellulosic
material in the first vessel. Introducing wash liquid,
rather than a large amount of white liquor to the bottom
of the first reactor vessel, reduces lignin precipitation
in the first vessel that might otherwise occur if larger
amounts of white liquor were added to the bottom of the
first reactor vessel.
[0019] The second reactor vessel may be a continuous
digester vessel, such as a vapor or steam phase digester.
The use of a vapor or steam phase digester should avoid
operating problems in the top of the second reactor
vessel, caused by gas formation during the hydrolysis.
The first and second reactor vessels may be substantially
vertical, have a height of at least 100 feet, an inlet in
an upper section of the vessel, and a discharge proximate
a bottom of the vessel. Heat energy added to the reactor
vessels may be pressurized steam at above atmospheric
pressure.
[0020] FIGURE 1 is a schematic diagram of an exemplary
chip feed and pulp processing system having a first
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reactor vessel 10 (hydrolysis reactor) and a second
reactor vessel 12, e.g., a continuous pulp digester. The
first reactor vessel includes an inverted top separator
14 that receives a slurry of cellulosic material and
liquid from a conventional chip feed assembly 15 via chip
feed line 33.
[0021] The chip
feed assembly 15 may include a wood
chip bin 16, such as the Diamondback Chip Bin sold by
Andritz Inc., connected to a double screw chip meter 18
and a chip chute 20. Hot water 24 is added via pipe 26
to the chips or other cellulosic material in the chip
chute 20 to form a slurry of cellulosic material. A
liquid surge tank 22 supplies the water to the chip tube.
Water may also be supplied directly to the chip tube
through pipe 23.
[0022] Separated liquid discharged from the top
separator 14 and extracted to pipe 27 may be mixed (see
valve 25) with hot water. The mixture flows through pipe
26 to the surge tank 22 and, via pipe 23, to the chip
tube 20. The
mixture of liquid discharged from the top
separator 14 and hot water 24 is controlled, using valve
25, to be at a temperature lower than the normal
hydrolysis temperature, e.g., preferably 170 C, of the
cellulosic material. The
temperature of the water and
liquid discharged from the top separator is preferably in
a range of 100 Celsius(C) to 120 C. By
temporarily
storing the mixture of water and liquor from the top
separator, the surge tank 22 may be used to provide
temperature control of the mixture of water and liquid
used to form the slurry of cellulosic material. For
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example, temperature control may be provided by adjusting
the relative amounts in the surge tank of liquid flowing
via pipe 27 from the top separator to the surge tank and
hot water 24.
[0023] To feed
chips to the first reactor vessel, the
slurry of cellulosic material is pumped via one or more
pumps 32 (such as the TurboFeed System as sold by
Andritz Inc., and pumps described in US Patents
5,752,075; 6,106,668; 6,325,890; 6,551,462; 6,336,993 and
6,841,042) to the top separator 14 of the first reactor
vessel. Other slurry feed systems, such as those using a
high-pressure feeders, may also be suitable.
[0024] The first
reactor vessel 10 may be controlled
based on either or both the pressure and temperature in
the vessel. Pressure
control may be by use of a
controlled flow of steam via steam pipe 74 or in addition
an inert gas added to the first reactor vessel. A
gaseous upper region 45 in the first reactor vessel is
above an upper level 44 of the chip column.
[0025] The
pressure from the gaseous phases assists in
forcing the cellulosic fiber material down and out of the
vessel at the bottom 56 discharge of the first vessel.
The latent pressure plus hydrostatic head should be
higher in the first reactor vessel 10 than in the second
reactor vessel 12 to assist in transporting the
cellulosic material discharged from the first reactor
vessel to the second reactor vessel. If the
latent
pressure and hydrostatic head is greater in the second
reactor vessel, a chip pump may be used between the two
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vessels to pump material from the first vessel to the
second vessel.
[0026] Steam 72
is supplied at a temperature above the
normal hydrolysis temperature, e.g., 170 C, to enable
hydrolysis to occur in the cellulosic slurry in the first
reactor vessel. The steam
is added in a controlled
manner that, at least in part, promotes hydrolysis in the
first reactor vessel. The steam
is added via lines 74
and 68 at or near the top of the first reactor vessel,
such as to the vapor phase 45 of the vessel. The steam
introduced to the first reactor vessel elevates the
temperature of the cellulosic slurry to at or above the
normal hydrolysis temperature, e.g., above 150 C.
[0027] The
cellulosic material slurry fed to the
inverted top separator 14 in the first reactor vessel may
have excessive amounts of liquid to facilitate flow
through the transport pipe 33. Once in the vessel, the
excess liquid is removed as the slurry passes through the
top separator 14. The excess liquid removed from the
separator is returned via pipe 27 to the chip feed
system, e.g., to the chip tube 20, and reintroduced to
the slurry to transport the cellulosic material to the
top of the first vessel. Hot liquid may be added at or
near the top separator 14 and gas phase 45 of the first
reactor vessel. The added liquid may be hot water 24
(piping not shown) or hot liquid extracted from the
extraction screen 48 in the first reactor vessel and
flowing through pipe 31 to the top of the first reactor
vessel.
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[0028] The top
separator 14 discharges chips or other
solid cellulosic material to a liquid phase (below upper
chip column 44) of the first reactor vessel. The top
separator pushes, e.g., by a rotating vertical screw, the
material from the top of the inverted separator 14 and
into the gas phase. The pushed out material may fall
through a gas phase 45 in the vessel and to the upper
chip column 44 of cellulosic material and liquid
contained in the first reactor vessel. The
temperature
in the gas phase (if there is such a phase) and in upper
region of the first reactor vessel 10 is at or above the
normal hydrolysis temperature, e.g., at or above 170 C.
The slurry of cellulosic material gradually flows down
through the first reactor vessel. As the material
progresses through the vessel, new cellulosic material
and liquid are added to the upper surface from the top
separator.
[0029]
Hydrolysis occurs in the upper region 46 of the
first reactor vessel 10, where the temperature is
maintained at or above the normal hydrolysis temperature.
The hydrolysis will occur at lower temperature, e.g.,
below 150 C, by the addition of acid, but preferably
hydrolysis occurs at high temperatures, above 150 C to
170 C, using only water and recirculated liquid from the
top separator of the first reactor vessel.
Hydrolysis
should occur substantially only in the upper region 46
above an extraction screen 48 or above a set of multiple
elevations of extraction screens 48.
[0030] To stop
hydrolysis as the cellulosic material
moves downward through the vessel 10 past the extraction
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screen 48, the temperature of the material is reduced to
below the hydrolysis temperature or acid in the
cellulosic material is removed from the first reaction
vessel through the extraction screens 48. Reducing
the
temperature and removing acids from the cellulosic
material may be used together or separately to suppress
and preferably stop hydrolysis.
[0031]
Hydrolysate is a product of hydrolysis. The
hydrolysate is removed with wash liquid and some other
liquids through the extraction screens 48 and fed to pipe
29 and flows to the flash tank 30. The hydrolysate, wash
liquid and other extracted liquids may be recovered or
recirculated to the chip feed system. The liquid in pipe
29 extracted from the first reactor vessel 10 and
directed to a flash tank 30 includes hydrolysate
extracted from the first reactor vessel. The flash tank
30 separates the hydrolysate laden liquid from steam.
The liquid from the flash tank is preferably at a
temperature below a hydrolysis temperature and more
preferably below 110 C. The
liquid with hydrolysate
flows from the flash tank to pipe 28 and the steam may be
returned via pipe 68 to an upper gaseous phase of the
first reactor vessel 10. A portion of the hydrolysate is
recovered by a conventional hydrolysate recovery system
70.
[0032] The steam
68 may be introduced to the vessel,
especially if the pressure in the vessel is lower than in
the flash tank. If the
pressure of the vessel is not
lower than the flash tank, the steam may be directed to a
chip bin, a heater for water and/or white liquor to be
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used in the process. Similar circulations of steam and/or
extracted liquids are described in US Patent 7,105,106
and US Patent Publication 2007-0000626.
[0033] The
liquids from the flash tank 30, including a
portion of the hydrolysate flows through pipes 28, 71 to
the chip slurry in the chip tube 20 and, via pipe 73, to
the liquid surge tank 22. The
amount of liquids with
hydrolysate added to the chip slurry in the chip chute 20
may be controlled to avoid excessive changes to the pH of
the chip slurry, e.g., to avoid making the slurry
excessively alkaline or excessively acidic. The addition
of liquid to the cellulosic material in the chip tube 20
assists in conveying the chip slurry material through the
chip pumps 32 and through the chip slurry pipes 33
extending between the chip chute 20 and the top separator
14 of the first reactor vessel 10.
[0034] A
counter-current wash zone 54 is in the vessel
below the extraction screens 48. The wash zone 54 is a
lower region of the vessel 10 below the extraction screen
48 and above the vessel bottom 56. The wash
liquid 50
flowing through the wash zone cools the cellulosic
material flowing through the wash zone to eliminate or at
least minimize continuing hydrolysis of the downwardly
moving chip stream in the wash zone 54. The wash liquid
is preferably 10 C to 70 C cooler than the hydrolysis
temperature, more preferably 20 C to 50 C cooler, and most
preferably 25 C to 35 C cooler.
[0035] The wash
liquid 50 flows in a counter flow
direction, e.g., an upward flow, to the downward flow of
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cellulosic material in the first reactor vessel. The
cool wash liquid 50 is pumped to the bottom of wash zone
from pipe 52 which connects to the bottom of the first
reactor vessel 10. The wash liquid pressure in pipe 52 is
sufficient to cause the wash liquid to flow upward (see
arrow designed 50) through the first reactor vessel 10 in
a counter-flow to the direction of cellulosic material
flowing downward through the vessel. The wash liquid is
removed at the extraction screen 48.
[0036] Chemicals 82, such as NaOH or essentially
sulfur free white liquor, may be added via pipe 84 to the
cool wash water flowing through pipe 52 prior to
introduction to the bottom of the vessel 10. The amount
of the added chemicals in the wash liquid may be an
amount of 0.01 percent (%) to 5 percent of the amount of
cellulosic material, e.g. wood, in the slurry flowing
through the vessel. The
amount of added chemicals is
preferably 0.1 percent to 1 percent of the cellulosic
material. The chemical(s) are added to the wash water to
suppress hydrolysis and remove hydrolysate, and
optionally to adjust the pH of the wash liquid. The
addition of the chemicals to the wash water results in
substantially more hydrolysate being extracted from the
cellulosic material flowing through the wash zone, that
would occur if the wash liquid was purely water.
[0037] As the wash liquid 50 interacts with the
cellulosic material in the wash zone and at or just above
the extraction screen 48, the liquid cools the cellulosic
material to below the hydrolysis temperature and washes
some chemicals out of the material. Preferably, the cool
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wash liquid, reduces the temperature of the cellulosic
material near the extraction screens 48 and in the wash
zone 54 to suppress and stop hydrolysis reactions in the
material. In
addition, as the hydrolyzed cellulosic
material moves below the extraction screens 48, it is
preferred that the material be at a pH level at which
lignin does not dissolve. The amount of wash liquid and
the chemicals in the wash liquid may be adjusted to cause
the pH level of the cellulosic material in the wash zone
54 to be within a predetermined pH range.
[0038] The
washed chips are discharged through the
bottom 56 of the first reactor vessel and sent via chip
transport pipe 62 to the top separator 57, e.g., an
inverted top separator, of the second reactor vessel 12,
such as a continuous digester. A pump 64 is optionally
used to assist in the transport of the cellulosic
material through pipe 62 from the first reactor vessel to
the second reactor vessel. Water and other liquids
remaining in the chips may be used to increase the liquid
to chip ratio in the cellulosic material flowing through
pipe 62 to assist in the transport of material through
the pipe 62 and to the top separator 56 of the second
reactor vessel.
[0039]
Additional liquid, from pipe 58, may be added
to the cellulosic material slurry in the transport pipe
62 or to the bottom of the first reactor vessel through
pipe 61. The additional liquid may be extracted from the
top separator 57 of the second reactor vessel 12. The
additional liquid may be recirculated by pumping (via
pump 59) and via pipes 58 and 61 to the bottom 56 of the
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first vessel. The
liquid in line 58 may be introduced
directly into the discharged stream of cellulosic
material in pipe 62 or via pipe 61 into the bottom 56 of
the first reactor vessel as part of the liquid used to
assist in the discharge of the chips form the first
vessel. A valve 63 directs liquid flow from pump 59 and
pipe 58 to pipe 61 or transport pipe 62. The
liquid
recirculated from the top separator 57 of the second
vessel should be relatively free of alkaline materials
and the pH control may regulated to ensure that the
recirculated liquid has an acceptable pH level before
being introduced into bottom of the first reactor vessel
or transport pipe 62.
[0040] Acid may
be added to the circulation pipe 62 to
assist in pH control of the cellulosic material being
transported from the first reactor vessel to the second
reactor vessel. If the pH of the cellulosic material in
the chip transport pipe 62 is above a desired pH level,
the addition of an acidic chemical into the pipe 62 or to
the bottom 56 of the first reactor vessel may be used to
decrease the pH in the cellulosic material.
[0041] A pH
monitor 78 may be used to sense the pH
level of the cellulosic material flowing from the first
reactor vessel to the second reactor vessel. If the
monitor 78 detects a pH level in the cellulosic material
above a desired pH range, a controller may cause an
acidic chemical to be added to the cellulosic material in
bottom 56 of the first vessel 10 or in the transport pipe
62. Additionally, if the monitor 78 detects a pH level
above the desired pH range, the controller may cause
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additional wash water to be introduced into the bottom 56
of the first vessel or to the pipe 62.
[0042] Steam
from the flash tanks 30, 66 may be may be
conveyed may be used, via pipe 68, to add heat to any of
the chip feed system 16, the first reactor vessel and a
heat recovery system 90. For example, the steam extracted
from the first reactor vessel 10 may be added to the chip
bin 16 to assist in the production of the slurry of
cellulosic material and for controlling the liquid to
wood ratio in the slurry. Before adding the steam to the
chip feed system, the steam may be checked to confirm
that it is substantially free of sulfur.
Preferably, no
sulfur containing chemical is added to the cellulosic
material or to any other material or liquid introduced
into the first reactor vessel 10. Sulfur in the first
reactor vessel 10 could undesirably result in sulfur
compounds in the vessels 10, 12 and in liquids extracted
from the extraction screen 48.
[0043]
Additional steam 72 may be added via pipe 74 to
the tops of the first reactor vessel 10 and to the top of
the second reactor vessel 12. The
additional steam may
provide heat energy for the reactor vessels.
[0044] Cooking
chemicals, e.g., white liquor 76, are
added to the top, e.g., to an inverted top separator 57
of the second reactor vessel 12. A
portion of these
cooking chemicals may be introduced to the circulation
line 58 extracting liquor from the top separator 57 and
adding liquor to the bottom of the first reactor vessel
or to the chip transport line 62. White liquor 76 is
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added to the top separator of the second reactor vessel
12 to promote mixing of liquor with the cellulosic
material in the separator and before the mixture of
material and liquor is discharged from the separator to
the second reactor vessel.
[0045] Monitoring of circulation line 58 may be
useful, including a pH monitor, to confirm that cooking
chemicals do not flow from the second reactor vessel 12
to the first reactor vessel 10 or to the transport pipe
62. The pH
in the circulation line 58 should remain in
the range of 4 pH to 10 pH, preferably in a range of 6 pH
to 10 pH, and more preferably a range of 6 pH to 8 pH.
If the pH in the circulation line 58 is high, additional
cool wash water 50 may be added to the bottom 56 of the
first reactor vessel or to the transport line 62. The
wash water 50 may be added to the bottom of the first
reactor vessel or the transport line 62 to assist in
pushing the slurry cellulosic material from the first
vessel to the top of the second reactor vessel.
[0046] The second reactor vessel 12 may be a
pressurized gas phase continuous digester vessel. The
liquid level in the second reactor vessel is below the
gas phase in the vessel and is sufficient to entirely
submerge the solids, e.g., chips, of the cellulosic
material. The liquid level in the second reactor vessel
may be as high as the upper rim of the top separator 57.
This high liquid level may be helpful to provide a quick
and thorough penetration of cooking chemicals into the
chips. Cooking in the second vessel is co-current.
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[0047] The
second reactor vessel 12, e.g., a cooking
or digesting vessel, may be a single vessel system with
multiple stages where the cellulosic material passing
through the first stage (upper elevation) is at a lower
temperature than the cellulosic material at other stages
(lower elevations). An optional cooking or digester
operation employs cooking the cellulosic material as soon
as the chips are introduced into the cooking liquor. Yet
another optional cooking or digester operation is cooking
the cellulosic material as it is introduced to the
cooking liquor and cooking the material at different
temperatures as the cooking process proceeds through the
second reactor vessel. For
example, the second reactor
vessel may have multiple cooking zones at different
elevations and each zone is maintained at a different
cooking temperature.
[0048] Heat recovery systems 90 and methods are
conventional and well know in pulping plants. For example
heat from the circulation streams, such as from the flash
tanks 66, may be recovered in heat exchangers or other
such heat recovery systems 90. The recovered heat from
the flash tanks may also be applied to pre-heat liquid,
such as wash filtrate 80 and white liquor 76, introduced
to the top of the second reactor vessel. This pre-heating
of liquids may be accomplished by using heat exchangers
to extract heat from the flash tanks and transfer the
heat to the liquids.
[0049] While the invention has been described in
connection with what is presently considered to be the
most practical and preferred embodiment, it is to be
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understood that the invention is not to be limited to the
disclosed embodiment, but on the contrary, is intended to
cover various modifications and equivalent arrangements
included within the spirit and scope of the appended
claims.
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